Taurine can be referred to as 2-aminoethanesulfonic acid and is one of the amino sulfonic acids found in the tissues of many animals. Taurine is an essential natural compound that promotes human neonatal development, brain development, and heart function. Taurine finds wide applications as a dietary supplement and as a pharmaceutical in the treatment of cardiovascular disease, elevated blood pressure, hepatic disorders, diabetes, and dermatological conditions. Taurine is used as a key ingredient in energy drinks to improve performance. In addition, taurine may be used as a plant growth stimulator to increase crop yield and plant biomass.
Taurine is currently produced in an amount of over 60,000 tons per year from either ethylene oxide or monoethanolamine. At the present time, most taurine is produced from ethylene oxide, following a three-step process: (1) the addition reaction of ethylene oxide with sodium bisulfite to yield sodium isethionate; (2) the ammonolysis of sodium isethionate to yield sodium taurinate; (3) the neutralization with an acid, i.e., hydrochloric acid and, preferably, sulfuric acid, to generate taurine and inorganic salts.
Although the ethylene oxide process is well established and widely practiced in commercial production, the overall yield is not very high, less than 80%. Moreover, the process generates a large waste stream that is increasingly difficult to dispose of.
The first stage of the ethylene oxide process, the addition reaction of ethylene oxide with sodium bisulfite, is known to yield sodium isethionate in high yield, practically quantitative, as disclosed in U.S. Pat. No. 2,820,818 under described conditions.
Therefore, the problems encountered in the production of taurine from the ethylene oxide process arise from the ammonolysis of sodium isethionate and from the separation of taurine from sodium sulfate.
U.S. Pat. No. 1,932,907 discloses that sodium taurinate is obtained in a yield of 80%, when sodium isethionate undergoes ammonolysis reaction in a molar ratio of 1:6.8 for 2 hours at 240 to 250° C. U.S. Pat. No. 1,999,614 describes the use of catalysts, i.e., sodium sulfate, sodium sulfite, and sodium carbonate, in the ammonolysis reaction. A mixture of sodium taurinate and sodium ditaurinate is obtained in a yield as high as 97%. However, the percentage for sodium taurinate and sodium ditaurinate in the mixture is not specified.
DD 219 023 describes detailed results on the product distribution of the ammonolysis reaction of sodium isethionate. When sodium isethionate undergoes the ammonolysis reaction with 25% aqueous ammonia in a molar ratio of 1:9 at about 280° C. for 45 minutes in the presence of sodium sulfate and sodium hydroxide as catalyst, the reaction products comprise 71% of sodium taurinate and 29% of sodium di- and tri-taurinate.
WO 01/77071 is directed to a process for the preparation of ditaurine by heating an aqueous solution of sodium taurinate at a temperature of 210° C. in the presence of a reaction medium. A mixture of sodium taurinate and sodium ditaurinate is obtained.
It is therefore concluded from the foregoing references that the ammonolysis of sodium isethionate invariably yields a mixture of sodium taurinate, sodium ditaurinate, and sodium tritaurinate. The percentage yield of sodium taurinate has not been more than 80%.
In order to obtain taurine from sodium taurinate, U.S. Pat. No. 2,693,488 discloses a method of using ion exchange resins involving a strongly acid ion exchange resin in hydrogen form, and then an anion exchange resin in basic form. This process is complicated and requires the use of a large quantity of acid and base to regenerate the ion exchange resins in each production cycle.
On the other hand, CN101508657, CN101508658, CN101508659, and CN101486669 describe a method of using sulfuric acid to neutralize sodium taurinate to obtain a solution of taurine and sodium sulfate. Crude taurine is easily obtained by filtration from a crystalline suspension of taurine after cooling. However, the waste mother liquor still contains taurine, sodium sulfate, and other unspecified organic impurities, which are identified as a mixture of sodium ditaurinate and sodium tritaurinate.
U.S. Pat. Nos. 9,428,450, 9,428,451, 9,573,890, and 9,598,357 overcome some of the problems in the known ethylene oxide process by inhibiting the formation of the byproducts of the ammonolysis reaction of alkali isethionate, alkali ditaurinate and alkali tritaurinate, and converting the byproducts into alkali taurinate. The overall yield of the cyclic process for producing taurine from sodium isethionate is increased to from 85% to nearly quantitative.
CN 104945289A and CN 105732440A describe recycling of the mother liquor, which contains sodium ditaurinate and sodium taurinate, during the ammonolysis of sodium isethionate in the production of taurine to increase the yield and to reduce discharge of waste.
U.S. Pat. No. 8,609,890 discloses a cyclic process of using isethionic acid or sulfur dioxide to neutralize alkali taurinate to producing taurine and to regenerate alkali isethionate. U.S. Pat. No. 9,108,907 further discloses a process of using isethionic acid prepared from ethanol to neutralize alkali taurinate to regenerate alkali isethionate. The aim is to reduce or eliminate the use of sulfuric acid as an acid agent in the production of taurine.
U.S. Pat. No. 9,061,976 discloses an integrated production scheme by using sulfur dioxide as an acid and by converting the byproducts of the ammonolysis reaction, alkali ditaurinate and alkali tritaurinate, to alkali taurinate. The overall production yield is increased to greater than 90% and alkali sulfate is eliminated from the production process. One drawback of this process is the use of gaseous sulfur dioxide, which imparts a slight smell on the product. Another significant drawback is that the taurine product from this process may contain trace amount of alkali sulfite which could be an allergen for certain people.
U.S. Pat. No. 9,593,076 discloses a cyclic process for producing taurine from isethionic acid in a high overall yield of greater than 90% to nearly quantitative, while generating no inorganic salt as byproducts. Similarly, CN 106008280A describes the use of isethionic acid to neutralize sodium taurinate and to regenerate sodium isethionate. However, the starting material, isethionic acid, is difficult to obtain commercially and is produced by a costly process of bipolar membrane electrodialysis of alkali isethionate.
U.S. Pat. No. 9,850,200 discloses a process for producing taurine by using an ammonium salt to react with alkali taurinates to yield taurine. In particular, ammonium bisulfite, ammonium sulfite, or their mixture is used to produce taurine and to regenerate a mixture of alkali bisulfite and alkali sulfite. Other suitable ammonium salts are selected from the group of ammonium sulfate, ammonium bisulfate, ammonium chloride, ammonium bromide, ammonium nitrate, ammonium phosphate, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium carbonate, ammonium bicarbonate, ammonium carboxylate, ammonium alkyl sulfonate, ammonium aryl sulfonate, and a mixture of two or more thereof.
CN 101717353A describes a process of preparing taurine by (1) reacting ethylene oxide with ammonium sulfite to yield ammonium isethionate and ammonia; (2) ammonolysis of the obtained product to ammonium taurinate; (3) acidifying with sulfuric acid to afford taurine. However, repeated attempts fail to produce any taurine under disclosed conditions.
JPS63243066 discloses a process of preparing taurine by reacting aziridine or ethyleneimine with an aqueous solution of sulfurous acid and adjusting the pH of the solution with a base. Because of limited solubility of sulfurous acid, the reaction is carried out under very dilute condition and the process is not economical.
JPH04352760 discloses a process of preparing taurine by absorbing gaseous aziridine with a solution of excess ammonium bisulfite or alkali bisulfite. Taurine or alkali taurinate is separated by distilling off water under vacuum and the product is isolated by washing with an alcohol.
JPH08268995 describes a cyclic process of preparing taurine from aziridine by first reacting aziridine with an excess of alkali bisulfite to form alkali taurinate, which is neutralized with sulfur dioxide to taurine and to regenerate alkali bisulfite. The direct contact of sulfur dioxide with taurine imparts a slight foul smell on the final product taurine.
Chen et al describe a method of preparing taurine by reacting aziridine with an excess of ammonium bisulfite (Zhejiang Chemical Industry, 2011, Vol. 42, No. 5, pp 5, 18-20). However, the method gives only a moderate yield of about 75% and a large amount of mother liquor that is difficult to dispose of.
CN 103613517A discloses a process for producing taurine by reducing 2-nitroethanesulfonate sodium salt to sodium taurinate, which is neutralized with sulfuric acid to taurine and sodium sulfate. The reduction is carried out preferably by hydrogenation in the presence of a catalyst such as Raney Ni or Pd/C.
CN 105693559A describes a process of producing taurine from sodium taurinate by replacing sulfuric acid with carbon dioxide to produce taurine and coproduce sodium bicarbonate, which is a useful commodity.
CN 106588704A discloses a cyclic process that improves the CN 103613517A process by first reacting 2-nitroethanol with ammonium bisulfite to form ammonium 2-nitroethanesulfonate, which is reduced to ammonium taurinate by catalytic hydrogenation. Sulfur dioxide is then used to neutralize ammonium taurinate to taurine and to regenerate ammonium bisulfite. One drawback of this process is the use of gaseous sulfur dioxide, which is obnoxious and imparts a slight smell on the product. Another significant drawback is that the taurine product from this process may contain trace amount of alkali sulfite which could be an allergen for certain people.
U.S. Pat. No. 4,444,694 discloses a process for the preparation of metal salts of 2-aminoethanesulfonic acid, which comprises the reaction of 2-oxazolidinone with alkali sulfite, alkali bisulfite, or their mixture. The reaction yield is usually high, greater than 90%. A major drawback of the process is the use of an acid to neutralize alkali taurinate, thus generating a large quantity of inorganic salt as byproduct.
It is an object of the present invention to overcome the disadvantage of the known processes for the production of taurine and to provide, in addition, advantages, which will become apparent from the following description.
It is another object of the present invention to disclose a process for the production of taurine from ammonium isethionate in a high overall yield (i.e., greater than 90% to nearly quantitative) without generating any inorganic salt as byproduct.
It is a further object of the present invention to disclose a process for producing taurine by thermal decomposition of ammonium taurinate to taurine and ammonia. Additional acid is eliminated from neutralizing ammonium taurinate, thus avoiding the formation of inorganic salt byproducts.
The starting material, ammonium isethionate, can be readily and economically produced by reacting ethylene oxide with ammonium bisulfite according to prior art, e.g., U.S. Pat. Nos. 5,646,320 and 5,739,365.
According to the process of the present invention, a solution of alkali isethionate or regenerated alkali isethionate, alkali ditaurinate, and alkali tritaurinate is mixed with an excess of ammonia and is subjected continuously to the ammonolysis reaction to form a mixture of alkali taurinate, alkali ditaurinate, and alkali tritaurinate, in the presence of one or more catalysts. After ammonium isethionate is added to the ammonolysis solution to form ammonium taurinate and alkali isethionate, excess ammonia and ammonia released from a thermal decomposition of ammonium taurinate are removed to obtain a crystalline suspension of taurine in a solution of alkali isethionate, alkali ditaurinate, and alkali tritaurinate. Upon the solid-liquid separation of taurine, the mother liquor is directly recycled to the ammonolysis step.
The advantage of using ammonium isethionate as a starting material becomes apparent in that no isolation of alkali salt as a byproduct is necessary after the separation of crystalline taurine from the mother liquor containing alkali isethionate, alkali ditaurinate, and alkali tritaurinate. Moreover, the final product, taurine, does not contain any inorganic salt, such as alkali sulfate or alkali halide, as impurity.